Treatment of cellulose during bleaching with agent capable of reducing carbonyl groups

ABSTRACT

The present invention relates to a method for treating a mixture containing cellulose, comprising at least one step of adding at least one agent capable of reducing carbonyl groups. The invention further relates to a specialty cellulose pulp, obtained by a method comprising treatment. Furthermore, the invention relates to the use of the specialty cellulose pulp according to the invention or the specialty cellulose pulp obtained by a method according to the invention for the production of cellulose derivatives or materials containing cellulose molecules, including but not limited to, cellulose ethers or cellulose esters. Cellulose derivatives obtained from the specialty cellulose pulp according to the invention display increased viscosity and/or improved brightness over cellulose derivatives obtained from specialty cellulose pulp not subjected to the inventive treatment.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject application is a continuation of U.S. Ser. No. 10/184,009,filed Jun. 26, 2002.

FIELD OF THE INVENTION

The present invention relates to a method for treating a mixturecontaining cellulose, comprising at least one step of adding at leastone agent capable of reducing carbonyl groups. The invention furtherrelates to a specialty cellulose pulp, obtained by a method comprisingsaid treatment. Furthermore, the invention relates to the use of thespecialty cellulose pulp according to the invention or the specialtycellulose pulp obtained by a method according to the invention for theproduction of cellulose derivatives or materials containing cellulosemolecules as a raw material, including but not limited to, celluloseethers or cellulose esters. Cellulose derivatives obtained from thespecialty cellulose pulp according to the invention display increasedviscosity and/or improved brightness over cellulose derivatives obtainedfrom specialty cellulose pulp not subjected to the inventive treatment.

BACKGROUND OF THE INVENTION

The method according to the invention can be employed for any mixturecontaining cellulose. Such mixtures include, by way of example, thetreatment of mixtures containing cellulose leading to specialtycellulose pulp which is used to illustrate the particular embodiments.However, this shall not be construed to mean that the method as claimedis only applicable to specialty cellulose pulp. For the purpose of thepresent invention, wherever applicable, the term “specialty cellulosepulp” is synonymous to the general term “pulp”.

Specialty cellulose pulp is used to manufacture a number of productsthat require physical and chemical properties not provided by the pulpused for the manufacture of standard paper, linerboard or cardboard.Specialty cellulose pulp therefore differs from the major portion ofcellulose pulp produced in the world today.

Any treatment that a mixture containing cellulose is subjected to, inparticular if it is to be used as specialty cellulose pulp, has to beperformed as to maintain the properties important to the end use, inparticular the integrity of the cellulose molecules themselves. One ofthe most important steps of treatment of a mixture containing cellulose,typically obtained from pulping wood, is a step of bleaching, or, inmost applications, a multi-stage bleaching process. Bleaching iscommonly achieved by treating a pulp slurry with chemicals that eitherremove colored compounds such as lignin, or alter the structure ofcolored compounds so that they are no longer colored. The extent towhich a specialty cellulose is bleached depends on the requirements forthe end-product manufactured from the specialty cellulose pulp.

The expert in the field typically understands a bleaching process as onethat increases the ISO brightness of a mixture containing cellulose.Furthermore, the expert in the field typically understands thatbleaching is achieved primarily by oxidation processes, i.e. bleachingof any mixture containing cellulose according to the prior art typicallyinvolves the application of an oxidizing agent. For environmentalreasons, common elemental chlorine bleaching using chlorine and/orhypochlorite is successively replaced by elemental chlorine freebleaching (ECF) processes, wherein chlorine dioxide replaces chlorineand/or hypochlorite. Totally chlorine free (TCF) bleaching useschlorine-free bleaching agents such as oxygen or peroxides. A method forthe non-chlorine bleaching of cellulose pulp using oxygen gas insophisticated treatment stages is described in, for example, U.S. Pat.No. 6,126,782. Other oxidizing agents are ozone or enzymes. JP-A 0 6 033390 for example discloses a method for using ozone to bleach pulp. Amain concern is to limit the oxidizing agent exposure time in order tolimit damage to the cellulose molecules.

In summary, bleaching techniques known to the expert in the fieldtypically involve oxidizing agents. The degree of ISO brightnessachievable by these methods of bleaching is limited to the extent thatstrong oxidizing agents (or highly concentrated oxidizing agents) whichwould result in a high degree of ISO brightness also tend to damage thecellulose molecules. In particular, strong or highly concentratedoxidizing agents tend to reduce the degree of polymerization (DP) of thecellulose.

A process known as “reductive bleaching” is used commonly forbrightening virgin mechanical pulps or pulp from recycled newsprint. Inthis process, a reducing agent, typically sodium borohydride, is used togenerate sodium hydrosulfite in situ from primary chemicals, i.e. thereducing agent is not used to reduce carbonyl-groups in the pulp but togenerate a bleaching agent. This process is disclosed, e.g. on pages 502through 504 of “Pulp Bleaching; Principles and Practice”, Dence andReeve (Eds.), TAPPI Press, 1996.

The use of reducing agents is also known in the context of late stagesof the treatment of cellulose pulps. For example, U.S. Pat. No.5,501,711 discloses the use of borohydrides to treat cellulose fabricthat already had been subjected to a bleaching treatment. Here, theapplication of reducing agents improves the dyeability of saidcellulosic fibers. The U.S. Pat. No. 6,217,621 relates to strippingtextile fibers, including cellulose acetates and other products obtainedfrom specialty cellulose pulp, of their dyes by using reducing agentssuch as borohydrides.

Another process involving the use of reducing agents in the context ofthe treatment of cellulose pulp is disclosed in U.S. Pat. No. 5,035,772.Said process involves treating a cellulose pulp that had already beenbleached with one or more reducing agents in order to reduce carbonylgroups on the lignin contained in the cellulose pulp. The processincludes adding a complexing agent together with the reducing agent andprocessing the mixture at a temperature not greater than 40° C. Thistreatment is followed by at least one further step: either a treatmentwith a chemical that will block the phenolic hydroxyl groups of thelignin and/or a treatment to convert short-wave light quanta to longwave light quanta. The object of the process according to the U.S. Pat.No. 5,035,772 is to prevent the yellowing with age of lignin containedin (high yield) cellulose pulps. The process is to be used withgroundwood pulp, refiner pulp, thermo-mechanical and chemical-mechanicalpulp for paper manufacture. Therefore, the U.S. Pat. No. 5,035,772 doesnot relate at all to the treatment of specialty cellulose pulp.

Finally, the possible use of borohydrides has been discussed in “PulpBleaching; Principles and Practice”, Dence and Reeve (Eds.), TAPPIPress, 1996, page 297, wherein a borohydride is mentioned as a possibleadditive to eliminate some of the loss in strength of paper induced bythe alkaline extraction.

The object of the present invention was to provide a method of treatinga mixture containing cellulose, preferably a mixture containingcellulose leading to specialty cellulose pulp, so that the degree ofbrightness or the viscosity of cellulose derivatives obtained therefrom,or both, is/are increased over the prior art. In addition, as a result,a specialty cellulose pulp as such, leading to cellulose derivativeswith increased viscosity and/or brightness was to be provided.

SUMMARY OF THE INVENTION

Surprisingly, it was found that this object could be achieved bytreating a mixture containing cellulose with at least one step of addingat least one agent capable of reducing carbonyl groups. This treatmentcan be performed at any stage of processing a mixture containingcellulose into specialty cellulose pulp once a mixture containingcellulose has been obtained from (i) chemically pulping wood(=“cooking”), (ii) chemically and mechanically pulping wood or (iii)unpulped cotton or (iv) any combination of (i) to (iii). The inventivetreatment is either the sole step of the entire process or is performedprior to bleaching and/or after bleaching or is performed as at leastone step of a multi-stage bleaching process.

The solution according to the invention is particularly surprising sincethe prior art does not teach that treating a mixture containingcellulose, i.e. treating a pulp, leads to cellulose derivativesobtainable from the treated pulp with improved viscosity and/orbrightness in comparison to a similar cellulose derivative obtained bythe same process but excluding the inventive step of adding at least oneagent capable of reducing carbonyl groups.

The present invention therefore relates to a method for treating amixture containing cellulose, comprising at least one step of adding atleast one agent capable of reducing carbonyl groups. The inventionfurther relates to a specialty cellulose pulp, obtained by a methodcomprising said treatment. Furthermore, the invention relates to the useof the specialty cellulose pulp according to the invention or thespecialty cellulose pulp obtained by a method according to the inventionfor the production of cellulose derivatives or materials containingcellulose molecules as a raw material, including but not limited to,cellulose ethers or cellulose esters. Cellulose derivatives obtainedfrom the specialty cellulose pulp according to the invention displayincreased viscosity and/or improved brightness over cellulosederivatives obtained from specialty cellulose pulp not subjected to theinventive treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1, the viscosity of the end product obtained from the inventivespecialty cellulose pulp, CMC, (vertical axis; units: mPa·s) is shown asa function of the limiting viscosity number of the specialty cellulosepulp as defined in the standard “SCAN-CM 15:99” (horizontal axis, units:ml/g).

In FIG. 2, the brightness of CMC powder (vertical axis, units: %brightness) is shown as a function of the intrinsic brightness of thespecialty cellulose pulp from which the CMC powder has been obtained bymeans of etherification (horizontal axis; units: % brightness)

FIG. 3 shows a typical particle size distribution of a CMC powder aftergrinding in the Fritsch Pulvarisette 19 knife mill as used in Examples1, 2 and 3. The horizontal x-axis represents the particle diameter inμm. while the vertical y-axis represents the volume of the correspondingparticles in %.

DETAILED DESCRIPTION OF THE INVENTION

A “mixture containing cellulose” according to the invention is anymixture that contains the glucose polymer of cellulose. No principallimitations exist as to the state of the cellulose source, i.e. it maybe solid, liquid, a suspension, slurry, paste or powder. Furthermore, nolimits exist as to the origin of the cellulose. In a preferredembodiment, the mixture containing cellulose is derived from cottonlinters and/or wood. In a particularly preferred embodiment, the mixturecontaining cellulose is derived from wood. Other cellulose sources, inparticular annual plants and/or biomass, (micro)biologically producedand/or derived cellulose, or cellulose from all types of cell walls areincluded as well.

Furthermore, no limitations exist with respect to the degree ofpolymerization of the mixture containing cellulose, as well as of anyproduct, such as specialty cellulose pulp and/or derivatives thereof,obtained from said mixture containing cellulose.

According to the invention, the mixture containing cellulose may bederived from raw materials that need to be pulped. In general, the term“pulping” refers to any process that separates the cellulose from the atleast one component that holds the cellulose together in the rawmaterial. For example, in wood, the cellulose is held together by ligninand hemicellulose in fibers. The pulping process is meant to separate,at least partly, the lignin from the carbohydrate moieties of thesefibers.

As to pulping processes, three different methods are commonly discerned:(i) chemical pulping (or “cooking”), (ii) mechanical (or “groundwood”)pulping and (iii) semi-chemical or chemical-mechanical pulping. In (i),the raw material, typically wood, is cooked in a “digester” at elevatedtemperatures with chemicals suited to break the bonds between thecellulose molecules and the lignin. In (ii), the raw material istypically pressed against a grinder which physically separates thefibers. The process (iii) refers to any combination of (i) and (ii). Inthe context of the present invention, no principal limitations exist asto how the pulping is to be performed, so long as the pulping results ina mixture containing cellulose that can be subjected to the treatmentaccording to the invention.

“Pulping” may not be necessary for all raw materials. For example, ifcotton linters are used as the mixture containing cellulose, no chemicalor mechanical pulping is necessary prior to any subsequent treatment,including the inventive treatment and/or bleaching.

A mixture containing cellulose that has been pulped as described above,is commonly referred to as “pulp”. In the context of the presentinvention, the term “pulp” is to be seen as more general than the term“mixture containing cellulose”, since the term “pulp” is meant to referto the mixture containing cellulose before the inventive treatment aswell as to the treated mixture, for example the specialty cellulose pulpas obtained after the inventive treatment. By contrast, the term“mixture containing cellulose” is only meant to describe the pulp beforethe treatment according to the invention.

Although the inventive method of treating a mixture containing cellulosewith the objective to obtain specialty cellulose pulp can be, inprinciple, performed with any mixture containing cellulose, the methodaccording to the invention is particularly effective, and thereforepreferred, when applied to a mixture containing cellulose derived fromwood. It is further preferred to pulp the wood chemically prior to theinventive treatment.

As far as wood as a source for the mixture containing cellulose isconcerned, both hardwood and softwood tree species may be used. Examplesof softwoods include but are not limited to: pines, in particularSouthern pine, White pine, Caribbean pine; Western hemlock; spruces, inparticular Norway Spruce, Sitka Spruce; Douglas fir or the like.Examples of hardwoods include, but are not limited to: gum, maple, oak,eucalyptus, poplar, beech, or aspen. Mixtures of two or more types ofsoft and/or hard wood are included as well.

Prior to the at least one step of adding at least one agent capable ofreducing carbonyl groups and/or prior and/or during and/or after anystep of pulping, the mixture containing cellulose may be optionallysubjected to any type of pretreatment. Such types of pretreatmentinclude but are not limited to enzyme treatments, mechanical refining,addition of additives, addition of complexing agents, treatment withdelignification and other catalysts, the removal of fines as well as anycombination of the aforementioned steps.

After the mixture containing cellulose has been prepared and/or obtainedand/or pretreated and/or treated with at least one agent capable ofreducing carbonyl groups (i.e. the inventive treatment), the mixture maybe subjected to a bleaching process. The term “bleaching” as used in thecontext of the present invention refers to any treatment of the mixturecontaining cellulose in which the degree of brightness after thebleaching is increased over the degree of brightness before bleaching.The term “ISO brightness” as used in the context of the presentinvention is defined in “ISO 2470-1999 —paper, boards and pulp—measurements of the diffuse blue reflectance factor (ISO brightness)”and refers to the brightness of the pulp. It is important todiscriminate this term from the term “brightness” as used in the contextof the present invention, which refers to the brightness of the endproduct, i.e. the derivative of the specialty cellulose pulp. A standardfor measuring said brightness according to the invention is given inExample 6. In principle, bleaching can be performed with any agentcapable of achieving the above mentioned objective.

At at least one stage of the processing of the mixture containingcellulose, obtained after pulping as has been described above, at leastone agent capable of reducing carbonyl groups (=reducing agent) is addedto the mixture containing cellulose. As a reducing agent in the contextof the present invention, every compound or mixture of compounds can beused that results in at least the partial reduction of at least a partof the carbonyl groups in at least one of the components containedwithin the pulp.

Carbonyl groups form a part of the molecular structure of the maincompounds in any mixture containing cellulose. As compounds are to benamed, by way of example, cellulose, hemicellulose, lignin and resins.The carbonyl groups according to the invention that are, at leastpartially, reduced in the inventive step of adding a reducing agent, mayeither be naturally present in the raw-material structures or may begenerated during processing or both. This is particularly true of themixtures containing cellulose used for the manufacture of specialtycellulose pulps derived from wood but is also the case, to a lesserextent, for pulp derived from other raw materials, such as pulp derivedfrom cotton linters.

In principle, any agent that at least partially reduces at least a partof the carbonyl-groups present in the mixture containing cellulose canbe used. Borohydrides are particularly preferred, while water-compatibleborohydride salts are further preferred. Such salts include but are notlimited to sodium borohydride, potassium borohydride, lithiumborohydride, sodium cyanoborohydride, sodium triacetoxyborohydride,sodium trimethoxyborohydride, tetramethylammonium borohydride,tetramethylammonium triacetoxyborohydride, tetraethylammoniumborohydride, tetrabutylammonium borohydride, tetrabutylammoniumcyanoborohydride, cetyltrimethylammonium borohydride,benzyltriethylammonium borohydride, Bis(triphenyl-phosphine) copper(I)borohydride, lithium aluminium hydride, dimethylamineborane (DMAB) andmixtures of at least two of these. Preferably, said reducing agents usedshould be water-soluble.

The reducing agent can be used by itself, in combination with otherreducing agents and/or in combination with stabilizers such as calciumhydroxide, magnesium bicarbonate or other mildly basic salts. Furtheradditional substances with other purposes may be added as well.

The reducing agents and/or additional substances may be added as asolid, a powder, a dispersion, suspension, emulsion or as a solution. Ina preferred embodiment, if borohydrides are used, they are used inpowder form or in the form of a standard solution, e.g. sodiumborohydride, which can be purchased as a 12 wt % solution in 40 wt %aqueous sodium hydroxide (e.g. Borol® from Rohm and Haas, Hydrafin™ fromFinnish Chemicals (Nokia) Ltd). All commercially available forms ofthese chemicals, as well as any chemical of this kind prepared in alaboratory, may be used to carry out the treatment of a mixturecontaining cellulose as disclosed here.

As has been mentioned above, the reducing agent may be added at anystage of the processing of the mixture containing cellulose. In apreferred embodiment, the reducing agent is added in at least one stepduring a multi-stage bleaching process.

In a particularly preferred embodiment, the at least one reducing agentis added, i.e. the inventive treatment is performed, at least as thelast stage or after the last stage of a multi-stage bleaching process.

The content of reducing agent added in at least one step may vary from0.01 to 500 g, preferably from 0.1 g·moles/ton mixture containingcellulose (dry basis) to 200 g·moles/ton mixture containing cellulose,depending on the chemical additives and the specific composition of themixture containing cellulose. In a preferred embodiment of theinvention, wherein sodium borohydride is added as the carbonyl-reducingagent and the sole purpose of the at least one step of the treatment isthe chemical reduction of carbonyl groups, the charge is between 0.3 and100 g·moles/ton mixture containing cellulose (on a dry basis).

In a further preferred embodiment, where sodium borohydride is added asthe carbonyl-reducing agent and an additional purpose of the at leastone step of the treatment is the delignification of the mixturecontaining cellulose, the charge is from 0.1 and 500 g·moles/ton mixturecontaining cellulose (on a dry basis), preferably from 0.1 to 80g·moles/ton.

In a preferred embodiment, the reducing agent is added as the at leastone agent in at least one stage of a multi-stage bleaching processand/or is added as at least one of at least two agents in at least onestages of a multi-stage bleaching process. In particular, the reducingagents can be added simultaneously during the addition of one or moreoxidizing agents, known to the expert in the field as the traditionalbleaching agents, at one or more stages during a multi-stage bleachingprocess. It is also conceivable, that adding reducing and oxidizingagents occur as alternating or subsequent steps, optionally separated bysteps of washing or otherwise treating the specialty cellulose pulp.

The reducing agent may be added together with any other agent orsubstance, so long as the at least one additional agent does not preventthe reducing agent to at least partly reduce at least a part of thecarbonyl groups. In a preferred embodiment, the pH achieved in the pulpupon adding the at least one reducing agent is below 12.

As far as the at least one oxidizing agent that may be added in amulti-stage bleaching process is concerned, any agent(s) known to theexpert in the field may be used. Examples of such oxidizing agents maybe selected from but are not limited the following group: chlorine,chlorine dioxide, hypochlorite, chlorite, oxygen, per-compounds such asperoxide, and ozone, as well as mixtures of two or more of theaforementioned substances.

Another agent that is commonly used during the bleaching process of aspecialty cellulose pulp, namely sodium hydroxide, may be added beforeand/or during and/or after any of the at least one step of bleaching asmentioned above. The main purpose of adding sodium hydroxide is theextraction of at least a part of the hemicellulose portion of the pulp,as well as, to some extent, regulation of the pH value. The amountand/or conditions under which the sodium hydroxide is to be added areknown to the expert in the art.

The aforementioned treatment of the mixture containing cellulose, can becarried out in any reaction device known to the expert in the field inthe context of pulp bleaching processes, for example in a bleachingtower, so long as the device is adapted to comply with health and safetyissues related to the use of the specific reducing agent(s) and/or anyother agent used. There are no limitations as to the design and/orfunction of the reaction device. For example, it may be a vessel or atube, operated in batch mode or continuously.

In a preferred embodiment, said reaction device will be a steel tower ofthe type known to the expert for commercial-scale cellulose pulpbleaching processes. In a further preferred embodiment, the device willbe of dimensions such that cellulose retention time under reactionconditions ranges up to 6 hours. Preferably, the device is of sufficientdimensions so that the reaction time ranges up to 3.5 hours. The actualsize of the reaction device is, inter alia, determined by the pulpproduction rate in a continuous process and, correspondingly, by thebatch volume in a batch process. Preferably, the reaction device isfitted with a fan capable of maintaining a level of hydrogen gas wellbelow the explosion limit since hydrogen gas may be generated duringtreatment with a borohydride. Any other method capable of keeping theamount of hydrogen in the reaction device below the explosion limit,such as hydrogen scavengers, controlled reactions of hydrogen or purgingthe reaction device with inert gases or mixtures containing inert gasesmay be used as well.

The mixture containing cellulose that is to be subjected to the at leastone step of the treatment according to the invention is pumped into theaforementioned reaction device. Preferably, it is pumped in the devicein the form of an aqueous slurry, having a content ranging from 0.1 to40 wt % of pulp based on dry mass of the mixture containing cellulose,preferably a content ranging from 1 to 25 wt %, further preferredranging from 5 to 15 wt %. The mixing of the slurry with the reducingagents may take place either inside or outside the reaction device, bymeans of a chemical mixer or any other means that produces homogeneousmixing. Any other chemicals added to the reactor device (e.g. for pHadjustment, bleaching purposes, hemicellulose extraction purposes or aschemical aids to the main chemical functions in the process stage) mayalso be mixed into the slurry inside or outside the reaction device.Preferably, any mixing takes place outside of the reactor device, asthis facilitates the formation of an homogeneous reaction slurry.

As far as the temperature at which the at least one reducing agent isadded to the mixture containing cellulose, is concerned, any temperatureis conceivable at which the at least one reducing agent at least partlyreduces carbonyl groups contained in the mixture. In case of anintegrated method comprising several process stages, in particularseveral stages of bleaching, the process temperature may be chosen to besuitable for all stages. In a preferred embodiment, the temperature willbe set as to optimize the integrated process, or, if necessary,regulated as to optimize each stage individually. In a further preferredembodiment, the multi-stage bleaching process is carried out at atemperature ranging from ambient temperature to 140° C.

In a further preferred embodiment and in particular if the chemicalreduction of carbonyl groups in the pulp is the primary object, thetemperature for the bleaching process ranges from ambient temperature to80° C., further preferred from 35° C. to 75° C. and yet furtherpreferred from 50° C. to 60° C. Furthermore, if the function of theinventive treatment also relates to delignification or to any form ofbleaching, in addition to chemical reduction of carbonyl groups, thetemperature of the reaction device is set to a value that allows thebest combination of bleaching action and chemical reduction of carbonylgroups. In a process stage of the process in which common bleachingchemicals known to the expert in the field are used, temperatures shouldpreferably range from ambient temperature to 80° C., further preferredfrom 30° C. to 70° C.

The heating of the reactor device is effected using heaters orheat-exchangers of the type known to the expert in the field.

The pH-value of the pulp slurry containing the previously describedactive chemical agents should be between 7 and 14. As for the case oftemperature, the pH of a preferred embodiment of the invention dependson the full process function of the process stage in question.

Most preferably, especially if sodium borohydride is used as thecarbonyl-reducing chemical, the pH should be maintained between 8 and14, in a preferred embodiment between 10 and 13 as to minimizedecomposition of the borohydride moiety and subsequent hydrogen gasrelease. A pH between 11 and 12 is particularly preferred. For the samereason, it is preferable to adjust the pH of the pulp slurry to asufficiently high pH prior to the addition of sodium borohydride. Thiscan be achieved by adding sodium hydroxide to the slurry at a pointbetween the previous process stage and the addition of sodiumborohydride. The amount of sodium hydroxide added depends on the pH ofthe mixture containing cellulose that is fed from the previous processstage.

Should the process stage where the invention is applied have theadditional function of delignification or other bleaching action usingcommon bleaching chemicals, the pH of the slurry in the reactor deviceshould preferably be set at a value from pH 8 to 14, wherein thespecific value chosen should best suit the optimal combination ofbleaching and chemical reduction of carbonyl groups in the pulp.

As a result of the treatment of the mixture containing cellulosedescribed above, in particular after treating said mixture in ableaching process comprising at least one stage of adding a reducingagent, a treated mixture containing cellulose is obtained. In thecontext of the present invention, this mixture is referred to as(treated) “specialty cellulose pulp”, i.e. as a pulp that has beentreated and can now be further processed to obtain specialty celluloseproducts. By way of example, one class of products obtainable fromspecialty cellulose pulp are cellulose ethers.

Said cellulose ethers, for example sodium carboxymethylcellulose,methylcellulose, methylhydroxyethylcellulose,methylhydroxypropylcellulose, hydroxyethylcellulose,hydroxypropylcellulose, ethylcellulose, and mixtures of at least twocomponents thereof, are used as additives in a large range of householdand industrial products. The main purpose of adding said celluloseethers to other materials lies in the possibility of controlling therheological properties of said materials. The viscosity of the celluloseether solution (henceforth termed cellulose ether viscosity), which isstrongly related to the degree of polymerisation (DP) of the cellulosein the pulp feedstock, is therefore one of the most important propertiesof cellulose ethers.

The upper limit of cellulose ether viscosity that can be obtained is perse limited by the upper limit of the cellulose DP in the specialtycellulose pulp from which the cellulose ether is derived, as well as bythe DP of the cellulose ether molecule following etherification.

Furthermore, in a large proportion of products containing celluloseethers, the appearance of the product is of high importance. Value isoften attached to a cellulose ether solid that is as white as possible,and to a cellulose ether solution that is as transparent as possible.The upper limit of brightness of cellulose ether that may be achieved isprincipally limited by the upper limit of the ISO brightness of thespecialty cellulose pulp from which it is derived.

The brightness achievable for commercial cellulose ether products isalso indirectly limited by the viscosity of the specialty cellulose pulpfeed-stock. This is because of a tradeoff in the production of specialtycellulose pulp between high ISO brightness and high cellulose DP in thespecialty cellulose pulp.

The specialty cellulose pulp as obtained after the inventive treatmentwith at least one agent capable of reducing carbonyl group and after anyother optional treatment, in particular after bleaching in a multi-stagebleaching process, can now be subjected to any step of post-treatment.

Of particular importance in this context is the removal of water fromthe specialty cellulose pulp. This can be achieved by filtering,pressing, drying at temperatures above room temperature, applying apressure that is below the partial pressure of water etc. Examples ofother post-bleaching steps, that may be applied either prior to, during,or after water removal, are, but are not limited to, removal of fines,enzyme treatments, and the addition of or treatments with chemicalagents to improve pulp processability characteristics. In a preferredembodiment, the specialty cellulose is present in the form of a solidsheet or powder, preferably obtained after at least one step ofpost-treatment.

The specialty cellulose pulp obtained as described above can besubjected to any step of further processing and/or derivatization. Ofparticular interest in the context of the present invention is anyprocess of forming cellulose derivatives. Cellulose ethers and/orcellulose esters are of particular importance in this context. Followingbleaching and post-treatment (including a possible stage of drying), thespecialty cellulose sheets are packed in a roll or sheets of a givenconfiguration determined by the cellulose derivative producer andtransported to the cellulose derivative producer. There, the sheets aretypically cut up or ground to a powder. In the case of cellulose ethers,the pulp pieces or powder is typically pre-treated with sodium hydroxideat below room temperature and reacted with the desired etherifyingagents in an oxygen-free environment at temperatures between 60 and 100°C. The cellulose ether product is washed free of salts, dried and oftenground to become the final product.

The viscosity of a cellulose ether as obtained from the specialtycellulose pulp as described above can be improved by at least 8-50%using the treatment as disclosed in this invention compared to the sameproduct not subjected to the at least one step of adding of at least onereducing agent during the processing of the specialty cellulose pulp.Characteristic advantages of the method according to the invention overthe prior art for etherification products obtained from the specialtycellulose pulp (in a substantially oxygen-free environment) areillustrated in FIG. 1 and FIG. 2.

In FIG. 1, the effect of the method according to the invention asapplied to a mixture containing cellulose from wood pulp is shown. Theviscosity of a sodium carboxymethyl cellulose (CMC), i.e. a cellulosederivative obtained via etherification from the specialty cellulose pulpaccording to the invention, is shown in FIG. 1. Here the viscosity ofthe end product, CMC, (vertical axis; units: mPa·s) is shown as afunction of the limiting viscosity number of the specialty cellulosepulp as defined in the standard “SCAN-CM 15:99” (horizontal axis, units:ml/g).

It can be seen that across a broad range of specialty cellulose degreeof polymerization (as represented by its limiting viscosity numbervalues), the corresponding viscosity of a 1% aqueous solution of CMCvaries linearly, irrespective of whether the mixture containingcellulose is treated according to the invention (open squares; solidline indicates least square fit) or if the mixture had not been treatedwith at least one reducing agent (open triangles). The data clearly showtwo distinct linear relationships for treated and untreated pulps.However, the viscosity of the CMC resulting from the treated specialtycellulose pulp is constantly about 200 mPa·s higher than the viscosityof the untreated, but otherwise same, mixture. These data provide clearevidence that specialty cellulose pulp that had not been treatedaccording to the invention does not reach its fullest degree ofpolymerization (DP) during etherification, as evidenced by asignificantly lower viscosity of the end product.

The large effect of the at least partial reduction of carbonyl groups ina specialty cellulose pulp on the brightness of its cellulose etherderivative was particularly unexpected. It has been found that thebrightness could be improved from 5% to 80%. This effect is shown inFIG. 2, where the brightness of CMC powder (vertical axis, units: %brightness) is shown as a function of the intrinsic ISO brightness ofthe specialty cellulose pulp from which the CMC powder has been obtainedby means of etherification (horizontal axis; units: % brightness). Itcan be clearly seen that the brightness of the treated specialtycellulose pulp (open squares) results in a brightness that is from 5 to20% higher than the corresponding brightness of the powder resultingfrom the untreated specialty cellulose pulp (open triangles).

The present invention also relates to the product of the inventiveprocess, i.e. to a treated specialty cellulose pulp obtainable by aprocess comprising the treatment of a mixture containing cellulose,wherein the treatment comprises at least one step of adding at least oneagent capable of reducing carbonyl groups. In particular, an integratedprocess may be employed. This process comprises at least the followingsteps:

-   -   (I) Chemical or chemical and mechanical pulping of a raw        material containing cellulose resulting in a mixture (I)        containing cellulose;    -   (II) treating an unpulped mixture containing cellulose or        treating a mixture (I) containing cellulose, wherein the        treatment comprises at least one step of adding at least one        agent capable of reducing carbonyl groups, resulting in a        treated specialty cellulose pulp (II);    -   (III) at least one step of post-processing of the specialty        cellulose pulp (II);

wherein the steps (I) and (III) are optional.

In a preferred embodiment, step (II) is performed as part of amulti-stage bleaching process. In a further preferred embodiment, thestep (III) of post-processing comprises at least one step of removingwater and/or of drying at temperatures above room temperature.

The specialty cellulose pulp as claimed in this invention can be usedfor any application for which specialty cellulose pulp is better suitedthan regular pulp used for the manufacture of paper or cardboard. Inparticular, all applications in which cellulose molecules alone or incombination with other materials can be used, are included.

By way of example but without the intent to limit the scope of theinvention, the following areas of use are mentioned: manufacture ofcellulose derivatives, in particular of cellulose ethers or celluloseesters; textile fibers, in particular viscose or high tenacity rayonyarn, non-woven fabrics; micro-crystalline cellulose, formulations forfood, in particular as edible diet food, pharmaceutical or cosmeticsapplications, technical filters, absorbing materials, fluff fibers,photographic papers, as an additive during plastic molding, the use ofsaid fibers in graft co-polymerisation, as a component in compositematerials, applications in packaging, paints, inks, thickeners, LCDscreens, high value specialty papers, laminates, battery separators,electrical circuits and the like.

EXAMPLES

In the following, working examples are used to illustrate the presentinvention. These examples, however are not meant to limit the scope ofthe invention as described above.

Example 1 Production of Specialty Cellulose Pulp According to theInvention and Subsequent Carboxymethylation

Norway spruce was cooked in batch using the acid sulfite process. Oncecooking was complete, the pulp had a mean kappa number (SCAN method C1:77) of 46. This pulp was transferred to a bleach plant typical of thedesign found in other bleach plants used in the manufacture of specialtyand other cellulose pulps.

In this first example the pulp bleaching process consists of fourdistinct, industrial scale stages, working continuously and in series,followed in series by a one stage, industrial-scale embodiment of theinventive treatment. All these stages, with the exception of theinventive treatments, are variations on bleach treatments common in thespecialty cellulose and other cellulose pulp industries, and are wellknown to one skilled in the art.

The main treatment at each of the five stages comprising this example iscarried out in stainless steel reactors commonly known as bleach towers,some of which are lined inside with chemically resistant materials,typical of those commonly used in the specialty cellulose and othercellulose pulp industries. Some additional but important procedures,such as washing, dilution, filtration, and chemical dosing, are carriedout either prior to the pulp entering, or after the pulp leaving thetowers. Details of the conditions inside each of the five towers and thebleaching, extraction chemical, and reducing agent dosages used in thisexample of the invention can be found in Table 1. TABLE 1 Details of theconditions inside the towers at each stage of processing in Example 1.Treatment stage Parameter Units Level E₀ NaOH Kg/ton pulp (dry) 57Temperature ° C. 95 Residence time Minutes 132 Consistency % 11.5 BorolSolution ® Kg/ton pulp (dry) 5 D₀ pH — 2 Temperature ° C. 15 Residencetime Minutes 43 Consistency 3.5 ClO₂ Kg/ton pulp (dry) 8.2 P₀ pH — 11Temperature ° C. 35 Residence time Minutes 126 Consistency % 10 H₂O₂Kg/ton pulp (dry) 2 D₁ pH — 2.5 Temperature ° C. 45 Residence timeMinutes 148 Consistency % 10 ClO₂ Kg/ton pulp (dry) 10 B pH — 11.6Temperature ° C. 53 Residence time Minutes 163 Consistency % 10 BorolSolution ® Kg/ton pulp (dry) 5

The term “consistency” as used in the context of the present inventionrefers to the dry mass of pulp in weight percent with respect to thetotal mass of pulp.

The first stage of this example of the invention, the stage (E₀), is analkaline extraction. In this stage, in addition to the main componentNaOH, a borohydride is added in the form of Borol Solution® (a 12 wt %solution of sodium borohydride in 40 wt % aqueous sodium hydroxidepurchased from Rohm and Haas). The dosage of Borol Solution® was 5kg/ton pulp (dry basis) which amounts to 15.9 g·moles of sodiumborohydride per ton dry pulp. This dosage was carried out immediatelyprior to the pulp entering the tower. Prior to the dosage of sodiumborohydride, the pulp was twice washed with deionized water, passed overa filter and press dewatered. Following pressing and prior to the sodiumborohydride dosage, 58 kg/ton pulp (dry basis) of sodium hydroxide wasdosed. Immediately prior to the pulp entering the E₀ tower, theconsistency of the pulp slurry was 11.5%. In the E₀ bleach tower, thetemperature was maintained at 95° C. The residence time of the pulpslurry at the base of the E₀ tower was 132 minutes. The alkalineextraction stage serves the purpose of hemicellulose removal and theborohydride serves as an additive to eliminate some of the loss instrength of paper induced by the alkaline extraction.

The second stage, namely the stage (D₀), is a chlorine dioxidetreatment. Prior to entering the D₀ tower, the pulp slurry was pressedand washed with deionized water, then washed a second time over afilter. Prior to the pulp entering the D₀ tower, the pH was adjusted to1.9, and 8.2 kg/ton pulp (dry basis) of chlorine dioxide was dosed.Following this chemical dosage, the consistency of the pulp slurry was3.5%. In the D₀ tower, the temperature was maintained at 15° C. Theresidence time of the pulp slurry in the D₀ tower was 43 minutes.

The third stage, namely the stage (P₀), is an oxidative bleaching stageusing hydrogen peroxide. Prior to entering the P₀ tower, the pulp-slurrywas washed with deionized water over a filter. The pH of the pulp slurrywas then adjusted to 10.9 using sodium hydroxide, and 2.0 kg/ton pulp(dry basis) hydrogen peroxide was added. The consistency of the pulpslurry was then 10%. In the P₀ tower, the temperature was maintained at35° C. The residence time of the pulp slurry in the P₀ bleach tower was125 minutes.

The fourth stage, namely the stage (D₁), is a chlorine dioxidetreatment. Prior to entering the D₁ tower, the pulp was washed withdeionized water over a filter. The pH of the pulp slurry was thenadjusted to 2.5 using sulphur dioxide, and 10 kg/ton pulp (dry basis) ofchlorine dioxide was dosed. In the D₁ tower, the temperature wasmaintained at 45° C. The residence time of the pulp slurry in the D₁tower was 148 minutes.

The fifth stage of this example of the invention, namely the stage (B),has the sole purpose of effecting chemical reduction of the carbonylgroups in the pulp, and is therefore the inventive borohydridetreatment. Sodium borohydride was dosed in the form of Borol Solution®.The dosage of Borol Solution® was 5 kg/ton pulp (dry basis), whichamounts to 15.9 g·moles of sodium borohydride per ton dry pulp. Thisdosage was carried out immediately prior to the pulp entering the Btower. Prior to the dosage of sodium borohydride, the pulp was washedwith deionized water over a filter and the pH of the slurry adjusted to11.5 using sodium hydroxide. Immediately prior to the slurry enteringthe B tower, its consistency was 10%. In the B tower, the temperaturewas maintained at 55° C. The residence time of the pulp slurry in the Btower was 163 minutes.

After leaving the B tower, the pulp slurry was washed with deionizedwater over a filter and the pH adjusted to 4. The pulp was thentransported to the drying section of the specialty cellulosemanufacturing process, where it was screened, washed in deionized water,and dried to a moisture content of 7%. The dried pulp was packed in aform (approx. 20 ton rolls) typical of finished product specialtycellulose that is ready for sale to other parties wishing to use this asraw material for a cellulose derivatization.

The properties of the finished specialty cellulose product can be foundin Table 2. TABLE 2 Selected properties of specialty cellulose pulpproduced according to Example 1. Brightness (ISO 2470-1999) (%) 82.00Limiting viscosity number (SCAN-CM 15:99) (ml/g) 1532 S18 (SCAN-C2:61)(alkali solubility of pulp; in %) 7.02

A 50 g sample of the specialty cellulose pulp manufactured using theinventive treatment described above and with properties as found inTable 2 was subjected to etherification yielding sodium carboxymethylcellulose (CMC).

The aforementioned specialty cellulose pulp sample was first groundusing a knife mill (Fritsch pulvarisette 19). The resulting specialtycellulose pulp powder had a mean particle size of 200 μm as measured bya Coulter LS 200 particle analyzer. 35 g of powder (weighed on a drybasis) and was subsequently etherified to carboxymethyl celluloseaccording to the carboxymethylation procedure described in Example 4.The 1 wt % solution viscosity and powder brightness of the resultantcarboxymethyl cellulose product was measured. The corresponding valuescan be found in Table 3. TABLE 3 Selected properties of CMC made fromspecialty cellulose pulp produced as per Example 1. The degree ofsubstitution of the CMC was 1.05. Viscosity of 1 wt % aqueous solution(mPa · s) 1840 Powder brightness (%) 71.1

Details of the procedures used for viscosity and brightness measurementsof CMC can be found in Example 4.

Example 2 Production of Specialty Cellulose Pulp and SubsequentCarboxvmethylation without the Inventive Treatment (Comparative Example)

In this example, exactly the same processing conditions as described inExample 1 (within the repeatability limits for an industrial scaleprocess) were used in the stages E₀, D₀, P₀ and D₁. However, noinventive treatment stage was applied in the form of the addition ofsodium borohydride or any other agent capable of reducing carbonylgroups at stage B. In this comparative Example 2, an oxidative bleachingstage using hydrogen peroxide (P₁) was directly substituted for theinventive stage B that had been applied in Example 1. Therefore, in thiscase, the pulp bleaching process consisted of five distinct, industrialscale stages, working continuously and in series. Details of theconditions inside each of the five towers and the bleaching andextraction chemical dosages used in this example of the invention can befound in Table 4. TABLE 4 Details of the conditions inside the towers ateach stage of processing in Example 2. Treatment stage Parameter UnitsLevel E₀ NaOH Kg/ton pulp (dry) 58 Temperature ° C. 95 Residence timeMinutes 134. Consistency % 11.5 Borol Solution ® Kg/ton pulp (dry) 5. D₀pH — 1.9 Temperature ° C. 15 Residence time Minutes 43 Consistency % 3.5ClO₂ Kg/ton pulp (dry) 8.9 P₀ pH — 11 Temperature ° C. 35 Residence timeMinutes 126 Consistency % 10 H₂O₂ Kg/ton pulp (dry) 2 D₁ pH — 2.5Temperature ° C. 45 Residence time Minutes 148 Consistency % 10 ClO₂Kg/ton pulp (dry) 10 P₁ pH — 11.6 Temperature ° C. 29 Residence timeMinutes 163 Consistency % 10 H₂O₂ Kg/ton pulp (dry) 2.5

The stage P₁, being the 5^(th) treatment stage during the bleaching ofthe specialty cellulose pulp in this example, was a hydrogen peroxidetreatment. The chemical reactor (bleach tower) used for this treatmentwas the same tower used as stage B in Example 1. Prior to entering theP₁ tower, the pulp was washed with deionized water over a filter and 2.5kg/ton pulp (dry basis) was then dosed simultaneous to the pH of theslurry being adjusted to 11.8 using sodium hydroxide. Immediately priorto the slurry entering the P₁ tower, its consistency was 10%. In the P₁tower, the temperature was maintained at 29° C. The residence time ofthe pulp slurry in the P₁ tower was 163 minutes.

The properties of the finished specialty cellulose product can be foundin Table 5. TABLE 5 Selected properties of specialty cellulose pulpproduced according to Example 2. Brightness (ISO 2470-1999) (%) 85.80Limiting viscosity number (SCAN-CM 15:99) (ml/g) 1574 S18 (SCAN-C2:61)(alkali solubility of pulp; in %) 7.00

A 50 g sample of the specialty cellulose pulp manufactured using thetreatment described above and with properties as found in Table 5 wassubjected to etherification to sodium carboxymethyl cellulose (CMC).

The aforementioned specialty cellulose pulp sample was first groundusing a knife mill (Fritsch pulvarisette 19). The resultant specialtycellulose pulp powder had a mean particle size of 200 μm as measured bya Coulter LS 200 particle analyzer. 35 g of powder (weighed on a drybasis) was subsequently etherified to carboxymethyl cellulose accordingto the carboxymethylation procedure described in Example 4. Relevantproperties of the resultant carboxymethyl cellulose product weremeasured, and their values are found in Table 6. TABLE 6 Selectedproperties of CMC made from specialty cellulose pulp produced as perExample 2. The degree of substitution of the CMC was 1.05. Viscosity of1 wt % aqueous solution (mPa · s) 1530 Powder brightness (%) 70.6

A comparison of the 1 wt % solution viscosity of the CMC obtained fromspecialty cellulose pulp obtained from Example 1 (Table 3) with thatfrom Example 2 (Table 6), reveals a substantially superior CMC viscosityof the CMC derived from pulp from Example 1 (20% higher). This isdespite the limiting viscosity number of the pulp from Example 1 being42 units lower than the pulp from Example 2.

Furthermore, despite the ISO pulp brightness of the specialty cellulosepulp from Example 1 being 3 units lower than the pulp from Example 2,the CMC powder brightness is of the same magnitude. These improvementsin CMC viscosity per unit pulp viscosity, and CMC brightness per unitpulp brightness, are indicative of the effect of the inventive treatmentdisclosed here.

Details of the procedures used for viscosity and brightness measurementsof CMC can be found in Examples 5 and 6.

Example 3 Post-bleaching Processing of Specialty Cellulose Pulp Usingthe Inventive Treatment and Subsequent Carboxvmethylation

In this example, a 50 g sample of the specialty cellulose pulpmanufactured using the treatment described in Example 2 and withproperties as found in Table 4 was treated as per an embodiment of theinventive treatment.

The 50 g pulp sample was partitioned into two equal halves of 25 g each.These samples were separately wet and torn into strips in 2.5 L ofdeionized water. The pulp suspension was then homogenized using adesintegrator (a steel rotor blade of 6 cm diameter rotating at 600 RPMfor 30 seconds). Both pulp suspensions were transferred into the samesealable plastic container. Into the pulp suspension was added 0.233 gof Borol Solution®, amounting to 5 kg of Borol Solution®/ton pulp (drybasis), or 15.9 g·moles of sodium borohydride per ton pulp (dry basis).The pH was then adjusted to 11.2 using sodium hydroxide.

The container containing the pulp suspension was sealed and shaken, andset in a water bath at a temperature of 55° C. for 180 minutes. The pulpsuspension was then washed twice with 5 1 of deionized water and driedto a moisture content of 7%. The limiting viscosity number and the ISObrightness of the pulp following the inventive treatment were found tobe unchanged from those of the pulp prior to applying the inventivetreatment. The pulp properties are therefore the ones found in Table 5.

This specialty cellulose pulp sample was then ground using a knife mill(Fritsch pulvarisette 19). The resultant specialty cellulose pulp powderhad a mean particle size of 200 μm as measured by a Coulter LS 200particle analyzer. 35 g of powder (weighed on a dry basis) wassubsequently etherified to carboxymethyl cellulose according to thecarboxymethylation procedure described in Example 4. Relevant propertiesof the resultant carboxymethyl cellulose product were measured, andtheir values are found in Table 7. TABLE 7 Selected properties of CMCmade from specialty cellulose pulp produced as per Example 7. The degreeof substitution of the CMC was 1.05. Viscosity of 1 wt % aqueoussolution (mPa · s) 1870 Powder brightness (%) 75.6

A comparison of the 1 wt % solution viscosity of the CMC obtained fromspecialty cellulose pulp obtained from Example 3 (Table 7) with thatfrom Example 2 (Table 6), reveals a substantially superior CMC viscosityof the CMC derived from pulp from Example 2 (22% higher). This isdespite having the same limiting viscosity number of the pulp.

Furthermore, the CMC powder brightness of the CMC obtained fromspecialty cellulose pulp obtained from Example 3 is 5 units, or 7%,higher than the CMC from example 2. This is despite the two pulps havingthe same ISO brightness values. These improvements in CMC viscosity perunit pulp viscosity, and CMC brightness per unit pulp ISO brightness,are indicative of the effect of the inventive treatment disclosed here.

Example 4 Carboxymethylation Procedure (As Used in Examples 1 Through 3)

-   -   Chemicals: Specialty Cellulose pulp, treated as per invention        (Borregaard ChemCell, Sarpsborg, Norway); iso-propanol (87 wt-%        in H₂O and 100 wt-%, ρ_(iso)=0.780 g ml⁻¹); N₂ (g); sodium        hydroxide (s); sodium monochloroacetate, 99% pure (s); acetic        acid; phenolphthalein; methanol (70 wt-%, ρ_(meth)=0.891 g        ml⁻¹); silver nitrate solution (0.1 M); deionized water.    -   Equipment: Refrigerator; analytical balance accurate to        1/100^(th) of gram; Parr-reactor complete with stirring        (cooled), external heating-element, internal cooling element, N₂        feed and temperature control; knife mill (Fritsch Pulverisette        19); measuring cylinders (100 ml, 500 ml); glass beaker; plastic        beakers (100 ml, 2 liter); graduated pipette (2-10 ml); glass        mixing rod; vacuum flask; ceramic vacuum funnel; black band        filter paper; watch glass (25 cm diameter); vacuum dryer.

The purpose of this example is to illustrate the process leading fromthe specialty cellulose pulp according to the invention to thederivative end product, in this case an etherified derivative. However,any other derivatization would be conceivable in this context, inparticular any process of esterification.

35 g (dry basis) of specialty cellulose pulp ground in the knife millwas introduced into the Parr-reactor. Immediately following this,precooled (5° C.) iso-propanol solution (448 ml 100% iso-propanol +40.7ml deionized water) was then poured into the reactor. The heating jacketwas fixed in place and the reactor sealed. The stirrer arm and itscooler were then connected. The nitrogen gas feed was turned on and theflow-rate regulated to 100 ml/min. The cooling water feeding both thestirrer and reactor cooling coil was turned on.

Stirring was set to 250 rpm and the temperature controller program (0°C. for 15 mins; 5° C. per min for 2 mins to 10° C. for 60 mins; 1° C.per minute for 50 mins to 60° C. for 60 minutes; 5° C. per minute for 6mins to 20° C. for 30 mins) was initiated.

After 15 minutes at 0° C., 22.0 g NaOH (NaOH-to-cellulose molar ratio is2.5) in 20.4 ml deionized water was introduced to the reactor. This isdone rapidly to avoid oxygen entering the reactor. Immediatelyafterwards, 78 ml iso-propanol (100%) was rapidly introduced. Thestirring speed was then increased to 500 rpm for a few seconds to attaina uniform mixture then decreased again to 250 rpm and the reactor wasleft for 60 minutes at 10° C.

51.3 g sodium monochloroacetate (MCA) (MCA-to-cellulose molar ratio is2.0) was mixed with 44 ml of iso-propanol (87%) and rapidly introducedto the reactor. The remaining, undissolved MCA residue was washed intothe reactor with a further 44 ml of 87% iso-propanol. The stirring speedwas then set at 500 rpm for a few seconds, then set back to 250 rpm. Thetemperature program then ensured the reactor contents were heated to 60°C. over 50 minutes and remained at this temperature for 60 minutes. Thetotal reaction time (with MCA) was therefore 120 minutes.

At the completion of 60 minutes at 60° C., the reactor contents wereneutralised (phenolphthalein indicator) using an acetic acid solution (5g acetic acid in 10 ml 87% iso-propanol). The reactor contents werevacuum-filtered and the product was first washed with 700 ml ofiso-propanol (87%), then four times with 700 ml of methanol (70%). Afterthe 5^(th) and final wash, the filtrate methanol was checked for beingchloride free using a few drops of AgNO₃. No precipitation of AgCl wasobserved meaning the product was sufficiently chloride free.

The washed product was dried in a vacuum drying cabinet at 60° C.overnight. The product was weighed to 1/100^(th) of a gram and itsmoisture content determined. The product was then ground to a powder ofmean particle size of 119 μm and a particle size distribution as shownin FIG. 1, in the knife mill.

Example 5 Procedure for Measurement of the Viscosity of a CMC Solution

2.00 g of CMC powder (dry basis) was dissolved in 200 ml of deionizedwater using a mechanical stirrer arm rotating at 200 rpm over the courseof 1 hour at room temperature. The 1 wt % CMC solution was thentransferred to a constant temperature water bath set at 20° C.

An Anton Paar Physica UDS 200 was used to perform the viscositymeasurement. The temperature bath on the instrument was first set to 20°C., and the spindle (MK 25/8) was mounted. The measurement program wasinitiated and 5.5 ml of 1 wt % CMC solution is placed in the receptorbelow the spindle. The spindle was set in the “measuring position” andsurplus sample was removed. Measurement was initiated and the dynamicviscosity in mPa·s was read as that reported from the machinemeasurement at a shear rate of 11.3 s⁻¹ as the spindle is on the way up.

Example 6 Procedure for Measurement of the Brightness of a CMC Powder

CMC powder was placed into a stainless steel receptor fitted with athreaded press and a glass plate that ensured a pressed, smooth powdersurface. After the CMC powder was placed on the glass plate, the presswas screwed into place, and the device was turned upside down anddisassembled. The powder then lay on the circular glass plate with thesmoothed powder surface facing upwards. The plate and powder sample wasthen placed in a Minolta CM-3630 apparatus, and the CMC powderbrightness read at a wavelength of 457 nm. Each sample was preparedtwice and the average of brightness readings is reported.

It should be noted that in order to obtain complete reproducibility ofthis procedure, any CMC powder analyzed must have the same or a verysimilar mean particle size and a particle size distribution as thatdescribed in FIG. 3. FIG. 3 shows a typical particle size distributionof a CMC powder after grinding in the Fritsch Pulvarisette 19 knife millas used in Examples 1, 2 and 3. The horizontal x-axis represents theparticle diameter in μm while the vertical y-axis represents the volumeof the corresponding particles in %.

1. A method for treating a mixture containing cellulose that has notbeen pulted or that has been pulped chemically or chemically andmechanically, characterized in that the treatment comprises at least onestep of adding at least one agent capable of reducing carbonyl groups.2. A method according to claim 1, character ized in that the at leastone agent capable of reducing carbonyl groups is added during at leastone step of a multi-stage bleaching process.
 3. A method for treating amixture containing cellulose that has not been pulped or that has beenpulped chemically or chemically and mechanically, wherein the methodcomprises: a) providing the mixture as an aqueous slurry; b) subjectingthe slurry to a multi-stage bleaching process; and c) during at leastone step of the multi-stage bleaching process, adding a sufficientamount of at least one agent capable of reducing carbonyl groups of thecellulose to produce at least a partially reduced cellulose; wherein atleast the terminal step of the multi-stage bleaching process comprisesthe addition of at least one agent capable of reducing carbonyl groupsof the cellulose to produce at least a partially reduced cellulose. 4.The method of claim 3, wherein the mixture containing cellulose isobtained from (i) chemically or chemically and mechanically pulping woodor is obtained from (ii) unpulped cotton linters or from any combinationof (i) and (ii).
 5. The method of claim 3, wherein the aqueous slurryhas a cellulose content of from 0.1 to 40% by weight based on the totalweight.
 6. The method of claim 3, wherein the agent capable of reducingcarbonyl groups is selected from the group consisting of borohydrides,dimethylamine borane (DMAB), lithium aluminum hydride (LiAlH), andmixtures thereof.
 7. The method of claim 3, wherein the sufficientamount of the agent capable of reducing carbonyl groups added to theslurry is from 0.1 to 200 gram·moles per ton of the mixture containingcellulose.
 8. The method of claim 3, wherein the step of adding at leastone agent capable of reducing carbonyl groups is performed attemperatures from ambient to 80° C.
 9. The method of claim 3, whereinthe step of adding at least one agent capable of reducing carbonylgroups is performed at a pH value from 8 to
 14. 10. The method of claim3, wherein the method further comprises performing at least one step ofpost-processing of the cellulose, wherein said post-processing isselected from the group consisting of de-hydration, filtering, drying,pressing, vacuum drying, enzyme treatment and removal of fines.
 11. Themethod of claim 10, wherein the post-processing step is a step ofdrying.
 12. The method of claim 3, wherein the multi-stage bleachingprocess is performed in batch mode or continuously.
 13. The method ofclaim 3, wherein the multi-stage bleaching process comprises adding atleast one oxidizing agent to the unprocessed cellulose prior to theterminal step of the multi-stage bleaching process.
 14. The method ofclaim 3, wherein an oxidizing agent is added concurrently with thereducing agent.
 15. The method of claim 3, further comprisingdelignification of the mixture containing cellulose.
 16. An improvedmethod for conducting a multi-stage bleaching process with an aqueouscellulose slurry where the cellulose that has not been pulped or thathas been pulped chemically or chemically and mechanically, wherein theimprovement comprises during at least one step of the multi-stagebleaching process, adding a sufficient amount of at least one agentcapable of reducing carbonyl groups of the cellulose to produce at leasta partially reduced cellulose and further wherein at least the terminalstep of the multi-stage bleaching process comprises the addition of atleast one agent capable of reducing carbonyl groups of the cellulose toproduce at least a partially reduced cellulose.
 17. The method of claim16, wherein the multi-stage bleaching process is performed in batch modeor continuously.
 18. The method of claim 16, wherein the multi-stagebleaching process comprises adding at least one oxidizing agent to theaqueous cellulose slurry prior to the terminal step of the multi-stagebleaching process.
 19. The method of claim 16, wherein an oxidizingagent is added concurrently with the reducing agent.
 20. The method ofclaim 16, further comprising delignification of the aqueous celluloseslurry.